190 Sargant and Arber.—The Comparative Morphology of 
root—forms a lateral xylem crescent. The single element px x ., which 
becomes visible in the peripheral gap between the lateral traces, is derived 
from the xylem bridge too. 
The vascular elements of the mesocotylar stele in Sorghum are thus 
derived from three distinct sources : 
1. The phloem of the median bundle (M in PI. X, Fig. 10), and most 
of its xylem, is plumular; for it is derived from the midrib of the first leaf 
and the plumular traces inserted on it above the node. 
2. The phloem of the two lateral bundles, and much of their xylem, is 
derived from the coleoptile bundles, and the numerous plumular traces 
inserted on them. The symmetry of each lateral bundle reproduces that of 
a coleoptile bundle; there are two phloem groups and a common xylem 
crescent in one as in the other. The single element px x . (PL X, Fig. 10) also 
comes from this source. 
3. A certain number of large xylem elements, forming part of the 
lateral crescents, are derived from the root-xylem and disappear at the 
node. In particular the two ‘ sentinel J vessels (rx., rx., in PI. X, Fig. 10), 
which divide the median from the lateral bundles, can be traced throughout 
the mesocotyl into the root. 
Comparison of Sorghum seedling with that of A vena. 
There is of course no doubt as to the complete homology of scutellum, 
coleoptile, and primary root in the two seedlings. The real question 
is whether the mesocotyl of Sorghum is homologous with that of Arena. 
If it is not, what member or members of a typical monocotyledonous seed¬ 
ling does it represent ? We have already explained that we consider the 
mesocotyl of Arena to represent a fusion of the cotyledonary stalk with the 
hypocotyl (ante, p. 165). If, on the other hand, the mesocotyl is morpho¬ 
logically equivalent in both seedlings, how account for the differences 
in vascular anatomy? 
Adopting the latter view, we believe that the mesocotyl of Sorghum is 
equivalent to the mesocotyl of Avena, and we have therefore to explain its 
anatomy on that assumption. The most obvious difference is that in 
Arena the scutellum trace joins the stele of the axis at the top of the 
mesocotyl; that is, at some distance above the apparent insertion of 
the scutellum. Between these two levels the scutellum trace runs upwards 
in the cortex of the mesocotyl; side by side with the stele, but quite 
distinct from it (Text-fig. 6, p. 166). But in Sorghum the scutellum trace goes 
straight to the stele as soon as it enters the axis; the real insertion appears 
to lie in the same plane with the apparent. Our explanation is that the 
fusion of stalk and hypocotyl has proceeded further in Sorghum than in 
Arena ; the cotyledonary trace in the former runs up to the plumular node 
within the stele, not outside it. 
